Use kernelManager.shutdownAll() in ikernel.spec.ts

packages/services/test/kernel/ikernel.spec.ts

@@ -57,8 +57,7 @@ describe('Kernel.IKernel', () => {
   });
 
   afterAll(async () => {
-    const models = await KernelAPI.listRunning();
-    await Promise.all(models.map(m => KernelAPI.shutdownKernel(m.id)));
+    await kernelManager.shutdownAll();
   });
 
   describe('#disposed', () => {